diff --git a/odoo/models.py b/odoo/models.py
index 68d14683446..17f82685ad8 100644
--- a/odoo/models.py
+++ b/odoo/models.py
@@ -472,7 +472,7 @@ class BaseModel(metaclass=MetaModel):
         * If :attr:`._name` is set, name(s) of parent models to inherit from
         * If :attr:`._name` is unset, name of a single model to extend in-place
     """
-    _inherits = {}
+    _inherits = frozendict()
     """dictionary {'parent_model': 'm2o_field'} mapping the _name of the parent business
     objects to the names of the corresponding foreign key fields to use::
 
@@ -519,7 +519,7 @@ class BaseModel(metaclass=MetaModel):
     as attribute.
     """
 
-    _depends = {}
+    _depends = frozendict()
     """dependencies of models backed up by SQL views
     ``{model_name: field_names}``, where ``field_names`` is an iterable.
     This is only used to determine the changes to flush to database before
@@ -699,8 +699,8 @@ class BaseModel(metaclass=MetaModel):
         cls._table = cls._name.replace('.', '_')
         cls._sequence = None
         cls._log_access = cls._auto
-        cls._inherits = {}
-        cls._depends = {}
+        inherits = {}
+        depends = {}
         cls._sql_constraints = {}
 
         for base in reversed(cls.__base_classes):
@@ -713,10 +713,10 @@ class BaseModel(metaclass=MetaModel):
                 cls._sequence = base._sequence or cls._sequence
                 cls._log_access = getattr(base, '_log_access', cls._log_access)
 
-            cls._inherits.update(base._inherits)
+            inherits.update(base._inherits)
 
             for mname, fnames in base._depends.items():
-                cls._depends.setdefault(mname, []).extend(fnames)
+                depends.setdefault(mname, []).extend(fnames)
 
             for cons in base._sql_constraints:
                 cls._sql_constraints[cons[0]] = cons
@@ -724,6 +724,12 @@ class BaseModel(metaclass=MetaModel):
         cls._sequence = cls._sequence or (cls._table + '_id_seq')
         cls._sql_constraints = list(cls._sql_constraints.values())
 
+        # avoid assigning an empty dict to save memory
+        if inherits:
+            cls._inherits = inherits
+        if depends:
+            cls._depends = depends
+
         # update _inherits_children of parent models
         for parent_name in cls._inherits:
             pool[parent_name]._inherits_children.add(cls._name)
@@ -2841,7 +2847,7 @@ class BaseModel(metaclass=MetaModel):
                     field.required = True
                 if field.ondelete.lower() not in ('cascade', 'restrict'):
                     field.ondelete = 'cascade'
-                self._inherits[field.comodel_name] = field.name
+                type(self)._inherits = {**self._inherits, field.comodel_name: field.name}
                 self.pool[field.comodel_name]._inherits_children.add(self._name)
 
     @api.model
diff --git a/odoo/tools/misc.py b/odoo/tools/misc.py
index de8fa82d96e..8e7fecab123 100644
--- a/odoo/tools/misc.py
+++ b/odoo/tools/misc.py
@@ -940,22 +940,32 @@ def clean_context(context):
     """ This function take a dictionary and remove each entry with its key starting with 'default_' """
     return {k: v for k, v in context.items() if not k.startswith('default_')}
 
+
 class frozendict(dict):
     """ An implementation of an immutable dictionary. """
+    __slots__ = ()
+
     def __delitem__(self, key):
         raise NotImplementedError("'__delitem__' not supported on frozendict")
+
     def __setitem__(self, key, val):
         raise NotImplementedError("'__setitem__' not supported on frozendict")
+
     def clear(self):
         raise NotImplementedError("'clear' not supported on frozendict")
+
     def pop(self, key, default=None):
         raise NotImplementedError("'pop' not supported on frozendict")
+
     def popitem(self):
         raise NotImplementedError("'popitem' not supported on frozendict")
+
     def setdefault(self, key, default=None):
         raise NotImplementedError("'setdefault' not supported on frozendict")
+
     def update(self, *args, **kwargs):
         raise NotImplementedError("'update' not supported on frozendict")
+
     def __hash__(self):
         return hash(frozenset((key, freehash(val)) for key, val in self.items()))